An apparatus is used in the prior art for regulating the connection and disconnection of a ringing signal to a called subscriber. The apparatus is described in the U.S. Pat. No. 4 145 577.
According to this patent the apparatus includes a two-way switching circuit connected in series to an AC voltage source which supplies an AC voltage at uniform intervals in the form of a ringing signal (hereinafter referred to as "call signal") to a called subscriber, a phase detector which detects the phase in the AC voltage and a control circuit connected to the phase detector for connecting and disconnecting the switching circuit synchronously with the output signal from the phase detector. The call signal to the subscriber starts at the AC voltage zero crossing and is interrupted when the current through the bell or buzzer has its zero crossing, thus preventing the occurence of undesired voltage transients.